Electron-rich triarylphosphines as nucleophilic catalysts for oxa-Michael reactions

Electron-rich triarylphosphines, namely 4-(methoxyphenyl)diphenylphosphine (MMTPP) and tris(4-trimethoxyphenyl)phosphine (TMTPP), outperform commonly used triphenylphosphine (TPP) in catalyzing oxa-Michael additions. A matrix consisting of three differently strong Michael acceptors and four alcohols of varying acidity was used to assess the activity of the three catalysts. All test reactions were performed with 1 mol % catalyst loading, under solvent-free conditions and at room temperature. The results reveal a decisive superiority of TMTPP for converting poor and intermediate Michael acceptors such as acrylamide and acrylonitrile and for converting less acidic alcohols like isopropanol. With stronger Michael acceptors and more acidic alcohols, the impact of the more electron-rich catalysts is less pronounced. The experimental activity trend was rationalized by calculating the Michael acceptor affinities of all phosphine–Michael acceptor combinations. Besides this parameter, the acidity of the alcohol has a strong impact on the reaction speed. The oxidation stability of the phosphines was also evaluated and the most electron-rich TMTPP was found to be only slightly more sensitive to oxidation than TPP. Finally, the catalysts were employed in the oxa-Michael polymerization of 2-hydroxyethyl acrylate. With TMTPP polymers characterized by number average molar masses of about 1200 g/mol at room temperature are accessible. Polymerizations carried out at 80 °C resulted in macromolecules containing a considerable share of Rauhut–Currier-type repeat units and consequently lower molar masses were obtained.


Reactions with acrylonitrile as Michael acceptor
Conversion of acrylonitrile was determined by integration of the signals deriving from the residual Michael acceptor (6.26-6.20 ppm, 1 H) and the CH2 group in the -position to the cyano group. NMR spectra of 1a-d are exemplarily shown in Figure S1, Figure S2, Figure S3, and Figure S4.
S3 Figure S2: 1 H-NMR spectrum (300 MHz, CDCl3) of 1b after 1 h reaction time; as catalyst TMTPP was used; conversion was calculated from the integral ratio of the peaks at 6.26-6.20 and the peak at 2.59.
S4 Figure S3: 1 H-NMR spectrum (300 MHz, CDCl3) of 1c after 24 h reaction time; as catalyst TPP was used; conversion was calculated from the integral ratio of the peaks at 6.26-6.20 and the peak at 2.61.
S5 Figure S4: 1 H-NMR spectrum (300 MHz, CDCl3) of 1d after 1 h reaction time; as catalyst MMTPP was used; conversion was calculated from the integral ratio of the peaks at 6.26-6.20 and the peak at 2.65 ppm.

Reactions with acrylamide as Michael acceptor
Conversion of acrylamide was determined by integration of the signals deriving from the residual Michael acceptor (5.61-5.57 ppm, 1 H) and the newly formed CH2 group. NMR spectra are exemplarily shown in Figure S5, Figure S6, Figure S7, and Figure S8.

Reactions with divinyl sulfone as Michael acceptor
Conversion of divinyl sulfone was determined by integration of the signals deriving from the residual Michael acceptor (6.42-6.37 ppm, 1H) and the CH2 group in β-position. NMR spectra are exemplarily shown in Figure S9, Figure S10, Figure S11, and Figure S12.
Mono/di ratios were determined from the integral ratio of the signals deriving from the protons adjacent to the sulfur atom (details see figures).